In the recovery of petroleum from subterranean reservoirs, it is usually possible to recover only a portion of the oil which is originally in place in the reservoir by the so-called primary recovery methods, that is, methods that utilize the formation energy for the production of the oil. A variety of supplemental recovery techniques, generally referred to as enhanced oil recovery processes, have been employed in order to increase the proportion of oil which is recovered. In these techniques, energy is supplied to the reservoir to provide a driving force for increasing the amount of oil which is recovered. Thermal recovery methods such as in situ combustion (fire flooding) are one type of process for recovering oil in this way and they are particularly suitable for the recovery of heavy or viscous petroleum deposits from tar sands and other reservoirs which cannot be economically produced in other ways.
The most common in situ combustion technique is the concurrent or forward burn process in which an injection well and a production well are driven into the subterranean, oil-bearing formation and the hydrocarbons in the formation are ignited around the injection well. An oxygen-containing gas such as air, oxygen-enriched air or substantially pure oxygen is then injected into the formation through the injection well to support burning of the hydrocarbons in the formation. A combustion front is established in the formation around the injection well and as the combustion process continues, this front advances through the reservoir in the direction of the production well. Preceding the combustion front is a high temperature zone, commonly referred to as a "retort zone" within which the reservoir oil is heated to effect a viscosity reduction and in which it is also subjected to various thermal processes such as distillation and cracking. Hydrocarbon fluids, including the heated crude oil and the distillation and cracking products of the crude, are then displaced towards the production well from which they may be withdrawn to the surface.
Carbon dioxide is formed as one of the products of combustion and as the combustion front advances through the formation, the generated carbon dioxide is displaced through the formation towards the production well and as it is displaced towards the production well, it dissolves in the reservoir oil, reducing its viscosity and consequently, improving its mobility. Thus, there are a number of different effects which contribute to the improved recovery of the oil during the process, including thermal viscosity reduction, distillation, cracking and carbon dioxide solution drive. Because these effects are particularly useful when the crude oil in the reservoir is a viscous, heavy oil, in situ combustion has commended itself for use in reservoirs, e.g. tar sands, which contain this type of oil.
In order to maximize the sweep efficiency of the process, it is desirable for the combustion front to advance through the formation in a uniform manner, preferably with the front remaining vertical during its progress from the injection well towards the production well. However, this ideal condition is unlikely to be achieved in practice for a number of reasons. First, the oxygen-containing gas tends to penetrate the formation in narrow streaks or fingers in which combustion takes place ahead of the main combustion front. If these fingers penetrate rapidly towards the production well they may provide a path along which oxygen can travel directly from the injection well to the production well without supporting any further significant degree of combustion in the formation. Also, these fingers promote instability in the combustion front which may make its progress less uniform and predictable than would be desirable. Another problem is that the combustion front tends to move faster at the top of the reservoir than at the bottom because the oxygen-containing gas, the combustion products and the hydrocarbons released by the process tend to be less dense than the crude oil in the reservoir; they therefore rise and travel across the top of the reservoir while the unaffected crude oil remains at the bottom of the reservoir, particularly in the region of the production well. Because these problems tend to decrease the sweep efficiency of the process, i.e., the efficiency with which the oil is displaced from the reservoir, it would be desirable to stabilize the combustion front and make its progress through the reservoir more predictable and uniform in character.